Small day 8 equine embryos cannot be rescued by a less advanced recipient mare uterus

摘要

Equine embryos tolerate an unusually large degree of negative uterine asynchrony (recipient mare up to 5 days behind the donor mare). By contrast, positive asynchrony of more than 2 days results in a high incidence of early embryonic loss (EEL). Day 8 embryos range in diameter from approximately 130-1300 mu m, with embryos smaller than 300 mu m reported to suffer an increased incidence of EEL. However, it is not known whether this reduced viability is due to intrinsically poor embryo quality, or to inadvertent recipient uterine stage-embryo (positive) asynchrony. To examine whether small embryos survive better in Day 4-5 recipients than in recipients with a more advanced uterine stage, the likelihood of pregnancy (PR) and EEL for 62 small (&300 mu m) and 215 larger Day 8 horse embryos were compared after transfer to recipients at different uterine stages (Days 4-5, 6-7 and 8-9) using logistic regression. Overall, EEL was higher (21.2%; P & 0.05) for small than larger embryos (7.1%). However, neither PR nor EEL were influenced by the recipient's uterine stage at the time of transfer (P& 0.1). The EEL for small embryos transferred into Day 4-5, 6-7 and 8-9 recipients was 20.8, 18.7 and 25.0%, respectively. We conclude that embryos recovered on Day 8 with a diameter &300 mu m are at increased risk of EEL due to reasons other than inadvertent positive asynchrony with the recipient mare's uterus. (C) 2018 Elsevier Inc. All rights reserved.